1. Field of the Invention
The present invention relates to an electric power generating method of a polymer electrolyte fuel cell which can be used as an electric car power supply or a domestic power supply system and uses hydrogen or hydrocarbon, such as alcohol, as fuel.
2. Description of the Related Art
To improve the performance of a polymer electrolyte fuel cell, a method has been reported, in which: projections and depressions are formed on the surface of an electrolyte membrane to increase a contact area between a catalyst layer and the electrolyte membrane. Japanese Laid-Open Patent Application Publication No. 2005-108822 discloses a method in which a projection-depression structure is formed on the surface of the electrolyte membrane by pressing against the surface of the electrolyte membrane a shaping die (template) which is made of a material having higher hardness than the electrolyte membrane and has the projection-depression structure. Japanese Laid-Open Patent Application Publication No. 2006-331720 discloses a method in which the projection-depression structure is formed on the surface of the electrolyte membrane by: preforming the projection-depression structure on the surface of a base material film which also serves to protect the electrolyte membrane; applying a polymer membrane formation material on the base material film and drying the polymer membrane formation material; and peeling off the base material film from the electrolyte membrane.
J. Am. Chem. Soc., Vol. 128, No. 39, 12971 (2006) discloses a method of: pouring a precursor liquid of an electrolyte material, which hardens by ultraviolet, onto a template film made of polycyano methyl acrylate and having a surface subjected to microfabrication; hardening the precursor liquid by ultraviolet; and peeling off the film from the electrolyte membrane.
FIG. 3 of Japanese Laid-Open Patent Application Publication No. 2005-108822 shows a flow chart for explaining respective steps of forming minute protrusions on the surface of the electrolyte membrane. According to this method, by pressing an electrolyte membrane 201 placed on a metal stand 204 by using a shaping die 203 having predetermined depression shapes, columnar minute protrusions 104 are formed on the surface of the electrolyte membrane. Two types of shapes are disclosed as the shape of each of the minute protrusions (shown as pillars in the table of Japanese Laid-Open Patent Application Publication No. 2005-108822). To be specific, these are a so-called rod-shaped minute protrusion (Japanese Laid-Open Patent Application Publication No. 2005-108822, FIGS. 1 and 2) having a diameter of about 0.3 μm and a length of 3 μm and a disc-shaped minute protrusion (Japanese Laid-Open Patent Application Publication No. 2005-108822, FIGS. 5, 7, and 8) having a diameter of 0.5 to 5 μm and a length (height) of 0.25 to 2.6 μm.
Disclosed as a method of manufacturing a fuel cell having the rod-shaped minute protrusions is a method of: forming a char layer on the surfaces of the minute protrusions by heating at 100° C. for two minutes under a nitrogen atmosphere or by sputtering; and depositing metal catalysts, such as platinum, on the char layer. However, it is considered that the char layer cannot be formed under the above conditions. Therefore, Japanese Laid-Open Patent Application Publication No. 2005-108822 does not disclose a feasible manufacturing method. In addition, since specific or quantitative explanations regarding results of a performance evaluation are not disclosed at all, the effectiveness of the rod-shaped minute protrusion is unclear.
Disclosed as a method of manufacturing an electrolyte membrane having the disc-shaped minute protrusions is an example in which such electrolyte membrane is produced by pressing a depression-type shaping die against a sulfomethylated polyether sulfone membrane. However, the heights of the minute protrusions are 0.5 to 1 μm, and the diameters of the minute protrusions are up to 2.6 μm. In contrast, the thicknesses of the catalyst layers formed on the surfaces of the electrolyte membrane having the minute protrusions are significantly thick, that is, the catalyst layer on a cathode side is 160 μm and the catalyst layer on an anode side is 55 μm. The effect of the existence or non-existence of the disc-shaped minute protrusions and the effect of the shape of the protrusion have been evaluated by comparing output densities of cells. However, the improvement of the output density is small, that is, twice at most.
Claims of Japanese Laid-Open Patent Application Publication No. 2005-108822 recite the diameter and height of the minute protrusion, but these are just general numerical values which can be used when constituting a fuel cell. Relations with the thickness and shape of the catalyst layer in an actual membrane electrode assembly (MEA) are not described, and experimental methods and data for demonstrating the effect are not disclosed.
FIG. 2 of Japanese Laid-Open Patent Application Publication No. 2006-331720 shows a flow chart for explaining respective steps of forming a minute projection-depression structure on the surface of the electrolyte membrane by casting. According to this method, by pressing a protective sheet 3a (3b) by two rollers 10 and 11, minute protrusions 4a on the roller 10 are transferred onto the protective sheet. Next, the electrolyte membrane is formed by ejecting a hydrocarbon polymer membrane formation material 12 onto the protective sheet from a nozzle 13; uniformly spreading the hydrocarbon polymer membrane formation material 12 by a doctor blade 14; and drying the hydrocarbon polymer membrane formation material 12. However, the shape of the minute projection-depression structure on the electrolyte membrane disclosed in Japanese Laid-Open Patent Application Publication No. 2006-331720 is the same as that disclosed in Japanese Laid-Open Patent Application Publication No. 2005-108822. Further, electric power generation conditions, especially a humidification condition of a supply gas, are not adequately disclosed.
Japanese Laid-Open Patent Application Publication No. 2005-085544 discloses, as a method of forming the projections, a method of causing a particulate polymer electrolyte to adhere to a flat membrane and a method of melting a part of the flat membrane. In addition, Japanese Laid-Open Patent Application Publication No. 2005-085544 discloses the electric power generation performance when using an anode gas and a cathode gas each having a relative humidity of 26%. Japanese Laid-Open Patent Application Publication No. 2005-085544 describes that the fuel cell including the electrolyte membrane having the projections outputs high voltage under the same conditions. However, Japanese Laid-Open Patent Application Publication No. 2005-085544 does not suggest the relation between the shapes of the projections and a preferable humidification condition (especially, see Japanese Laid-Open Patent Application Publication No. 2005-085544, paragraphs 0020 and 0036).
J. Am. Chem. Soc., Vol. 128, No. 39, 12971 (2006) discloses that: minute insertion portions (depressions) arranged regularly and each having a size of 3 μm×3 μm are formed on the surface of the electrolyte membrane; and by gradually increasing the depth of each insertion portion to 1.4, 1.9, and 3.7 μm, the performance improves at the time of low current density but deteriorates at the time of high current density. However, J. Am. Chem. Soc., Vol. 128, No. 39, 12971 (2006) does not disclose the desirable shape of the minute structure on the surface of the electrolyte membrane and the humidification condition of the supply gas.
Another literature related to the present invention is Japanese Laid-Open Patent Application Publication No. 2008-004486.
Each of Japanese Laid-Open Patent Application Publication Nos. 2005-108822 and 2006-331720 and J. Am. Chem. Soc., Vol. 128, No. 39, 12971 (2006) discloses that the improvement effect of the output density can be obtained by forming the minute protrusions on the electrolyte membrane to increase the surface area. However, which one of various parameters regarding the shapes of the depressions and projections dominantly affects the performance of the fuel cell is not disclosed in each literature. In addition, the degree of improvement of the output density disclosed in each literature is low, and a large effect which can change the practical value of the fuel cell is not achieved.
The present invention was made in view of the above circumstances, and an object of the present invention is to provide an electric power generating method of causing a polymer electrolyte fuel cell including an electrolyte membrane having a depression-projection structure to achieve an excellent electric power generation performance.